a. Field of the Invention
The present invention relates generally to methods and apparatus for cleaning particulate filters, and, more particularly, to a method and apparatus for effectively cleaning diesel particulate filters utilizing pressurized air applied to the ends of the filter in opposite directions.
b. Related Art
Clean air laws and regulations increasingly require that diesel engines be fitted with diesel particulate filters (referred from to-time-to time hereinafter by the abbreviation “DPF”) to remove particulate matter from the exhaust before it enters the atmosphere. While perhaps most well known with respect to the diesel engines found in trucks and buses, such regulations increasing apply to railway locomotives, ferries and other marine vessels, and other pieces of equipment, which therefore must likewise be fitted with diesel particulate filters.
The great majority of diesel particulate filters are axial-type filters constructed of ceramic media. FIG. 2 shows the body of an exemplary DPF, with the metal casing removed for clarity. As can be seen, the filter body A is (in this example) cylindrical in shape, with first and second end faces B, C and a multiplicity of bores or cells D somewhat resembling a honeycomb structure (although normally with square rather than hexagonal holes), with the bores being generally aligned with the axis of the main exhaust flow. As can be seen in FIG. 3 the bores are alternatingly open on one end and blocked on the other, so that each cell having an open end on the “dirty” side (C) of the filter body and a blocked end on the “clean” side (B) is adjoined by cells having open ends on the “clean” side and blocked ends on the “dirty” side, and vice versa. Unfiltered exhaust therefore enters those cells having open ends on the “dirty” side of the filter, as indicated by arrow E and then passes laterally through the filter medium (arrows F) into the cells having open ends on the “clean” side, from which the filtered exhaust is then discharged (arrow G).
In so doing, the particulate material H in the exhaust (which may be carried in unburnt hydrocarbons) is captured on the walls of the cells that open to the “dirty” side of the filter. With continued operation of the engine the captured particulate matter accumulates, typically building up progressively from the closed end of the bores towards the open ends at the “dirty” side of the filter. As a result, the DPF eventually becomes choked, restricting the ability of the exhaust flow to pass through it and thereby decreasing the engine efficiency; ultimately, if left unchecked, plugging of the DPF can lead to serious engine damage.
Operating guidelines provided by engine manufactures and others therefore specify that the DPF must be removed and cleaned at certain intervals, stated, for example, in terms of operating hours or backpressure measurements. Since diesel engines commonly operate for long hours or even continuously in a commercial environment, DPFs must therefore be removed and cleaned on a fairly frequent basis. This represents a very significant undertaking in the case of fleet operators, who may be running hundreds or even thousands of engines. Moreover, because the cleaning intervals do not always coincide with visits to a central maintenance facility, or the operator may lack such a facility, the filters must frequently be cleaned at truck stop service garages or similar, geographically distributed facilities.
Heretofore, however, the actual equipment and methods used to clean DPF at such facilities have been largely unsatisfactory, in terms of efficiency or effectiveness or both. For example, in many instances the cleaning is performed on a strictly manual basis, with a person blowing compressed air from a hose against the “clean” side of the filter body; not only is this practice exceptionally labor intensive and inefficient, it in fact fails to remove much of the accumulated particulate matter and leaves a significant portion of it in the filter; as a result, the DPF is left partially clogged after cleaning, not only reducing the time before the next cleaning will be needed but also tending to shorten the total life of the filter. Furthermore, excessive manual handling of the DPF increases the opportunity for damaging to the relatively fragile ceramic body of the filter, which typically costs $4,000-$5,000 to replace.
In addition of manual cleaning, several at least partially mechanized/automated systems have been developed for cleaning DPFs or similar filters. For example, U.S. Pat. No. 7,025,811 (Streichsbier et al.) shows an apparatus in which the DPF is sealed to a base so that a suction is applied to the “dirty” side of the filter, and an air nozzle is played across the “clean” side of the filter automatically, either by moving the nozzle only or both moving the nozzle and rotating the filter. Although more efficient and less labor intensive, this system is little or no more effective at actually removing the particulate matter from the DPF than the manual process described above.
The approach of applying pressure (“blow”) to the clean side of the filter and suction (“suck”) to the other has been all but universal in prior machines developed to clean axial filers. The Streichsbier approaches described in the preceding paragraph is an example of one type, another being to apply a flow or pulses of air to the “clean” side of the filter, while simultaneously applying suction to the other end using a similar fitting. These machines in turn virtually all demonstrate marginal or poor cleaning efficiencies; for example, it is believed that a “pulsed” system of the type described is able to remove only about 65-80% of the accumulated particulate material at best: Applicant hypothesizes that this poor performance is due at least in part to the pressurized air to escape through a few cells once they have been cleared and offer a low-resistance path, leaving the particulate in the remaining cells more-or-less undisturbed.
Another filter cleaning apparatus is shown in U.S. Pat. No. 4,808,234 (McKay et al). In this device a filter is clamped between two end plates and rotated on a horizontal axis, while nozzles on a pair of elongate tubes are moved over interior and exterior surfaces of the filter. This apparatus is capable of achieving good cleaning results but by its nature it is limited to use with radial-type, hollow-core filters (usually paper), such as those commonly used in air filters for industrial facilities, and is incapable of functioning with an axial-type diesel particulate filter.
Another deficiency of prior filter cleaning machines in general is an inability to assess or determine the point at which cleaning of a particular filter is substantially complete. The typical approach has been to simply set the machine to continue cleaning for a particular period of time, usually an average determined on an empirical basis. In actuality, however, individual filters differ tremendously in terms of the amount of cleaning required, based on operating loads/conditions of the engine, fuel types, age/condition of the DPF, operating hours since last cleaning, and other factors. Consequently, simply cleaning all filters for a particular amount of time, without being able to verify the extent to which the process has actually been completed, can result in less than the maximum amount of particulate being removed in some cases and excessive, inefficiently long periods of cleaning in others.
Yet another deficiency, common to existing automated filter cleaning apparatus, is an inability to easily accommodate filters of different sizes and shapes. Although the majorities DPFs are presently cylindrical in shape, some are square/cuboid, as in the case of those used in some locomotives, or have other shapes. Moreover, even the cylindrical DPFs vary is size, in terms of both diameter and length, depending on manufacturer, engine model/size, and so on. Consequently, unless a facility is dedicated to servicing a single type of filter, an inability to accommodate DPFs of varying sizes and shapes in a rapid and efficient manner represents a serious drawback.
Another factor tending to reduce the efficiency of existing DPF cleaning facilities is the inability to quickly and effectively identify damaged/failed filters (e.g., filters with failed cells or broken media) and segregate them from the cleaning process. The conventional technique for inspecting filters for damage involves using a bore scope to examine individual cells, which is a laborious and time-consuming procedure which must be undertaken separately from the cleaning process itself. Inspecting the filters prior to cleaning this greatly slows the overall process, but if left undone the cleaning process may be wasted on damaged filers and moreover a possibility exists that damaged filters may remain undetected and returned to use.
Accordingly, there exists a need for an apparatus and method for cleaning diesel particulate filters in a rapid and efficient manner with minimal manual involvement. Furthermore, there exists a need for such an apparatus and method that effectively removes the great majority of accumulated particulate matter from the DPF, so as to both maximize time between cleanings and extend the service life of the filter. Still further, there exists a need for such an apparatus and method that permits assessment of progress of the cleaning process so as to be able to determine the point at which the process is substantially complete. Still further, there exists a need for such an apparatus and method that can accommodate different sizes and shapes of filters in a convenient and rapid manner. Still further, there exists a need for such an apparatus and method that reduces the possibility of physical damage to the ceramic media of the filters. Still further, there exists a need for such an apparatus and method that allows rapid identification of filter having damaged media without the need for a separate, time-consuming examination process.